1. Field of the Invention
The present invention relates to ice detectors of the vibrating type, and in particular detectors having a small sensing surface which does not intrude substantially into the air stream being sensed and which may be made to conform to the external shape of the mounting surface.
2. Description of the Prior Art
In the prior art a vibrating type ice detector which changes the natural frequency upon accumulation of ice on the sensing probe is shown in U.S. Pat. No. 3,341,835. This patent has a tube supported substantially at a midpoint, which forms a node of vibration, and the tube is driven with a coil to vibrate axially between the node and its opposite ends at its natural frequency. The vibration is a minute lengthening and shortening of the two tube sections between the node and the respective ends. The frequency of vibration is sensed by a feedback coil. Ice accumulation on the exposed portion of the probe causes a change in natural frequency, as opposed to a dampening of amplitude of vibration, and this change in frequency is sensed as an indication of ice accumulation.
The circuitry used for driving the tube to its natural frequency, that is setting up longitudinal vibration between a mounting node and opposite ends of the tube, operates on the same principle as in the present application and the principle of sensing the change in frequency caused by ice accumulation is also the same. However, in U.S. Pat. No. 3,341,835 the probe protrudes substantially into the air stream, causing drag and the possibility of mechanical damage, and also forming a radar reflective target.
In the present device, the sensing surface is non-intrusive, and incorporates different physical principles which result in a change in frequency of vibration of the tube.
A mechanical displacement type ice detector sensor using a diaphragm is shown in U.S. Pat. No. 2,414,756. In this particular instance the diaphragm does not protrude into the air stream, as such, but the sensing is done by driving the diaphragm to deflect in opposite directions. Operation is carried out by measuring the dampening of displacement occasioned by ice accumulation. Frequency sensing is not used, and the structure disclosed in U.S. Pat. No. 2,414,756 has to be quite large so that there is a substantial change in mass of the system when ice accumulates on a diaphragm before accurate output readings can be obtained. This then has the problem of large size and low sensitivity to small accumulations of ice, and there is a requirement that there be substantial displacement of the diaphragm. Further, shaping this type of diaphragm to conform to the shape of the surface in which the ice detector is mounted is not practical in that the diaphragm has to be substantially planar for operation.
A frequency sensitive ice detector which has a large area, tapered disc covered with a fabric that is exposed to the formation of ice is shown in U.S. Pat. No. 2,800,647. In this patent an assembly stack of crystal plates forms a vibrating column supporting the disc, and the changes in resonant frequency of the vibrating column are sensed for determining ice accumulation.
The fabric outer cover forms an ice collecting surface, and the mass of ice on the surface is sensed by the vibrating system, due to natural frequency changes because of the change in mass on the fabric.
It is pointed out that the fabric area in this patent can form a smooth continuation of adjacent portions of the extended mounting surface. However, the sensor requires quite a large surface area; the use of fabric, which is not suitable for high performance aircraft; and a complex mounting assembly.
U.S. Pat. No. 3,240,054 also shows an ice detector that has a surface that collects ice, and depends upon the vibrating of an elastic diaphragm which changes in stiffness upon ice accumulation.
The device shown depends on the flexural stiffness of the entire diaphragm and diaphragms cannot be shaped to conform to a curved surface. U.S. Pat. No. 3,240,054 does, however, teach that the stiffness of ice can change resonant frequencies of elastic members that are being used for ice detectors.
An exposed diaphragm used for sensing ice formation is shown in British Specification No. 506,843, but again the diaphragm is motion sensitive, and the dampening of displacement by the mass of ice accumulated is necessary for providing an output.
Other non-intrusive ice detectors have been developed which depend upon sensing of the mass of ice, but one of the problems in having a very small sensor surface is that the mass of the ice is not very great in a small area, until accumulation is substantial, and thus the ability to sense small accumulations to give an early indication of ice formation is reduced.